This invention relates to a ballistic modifier for use in a double base plastisol rocket propellant in which a burning rate characterized by a plateau effect is desired.
The burning rate of a propellant at a given temperature is in direct relationship to the pressure to which it is exposed. Mathematically the relationship is expressed as: R = cp.sup.n where R is the burning rate, p is the pressure and c and n are constants determined by the type of propellant used.
Where n equals zero or some negative value, the burning rate will exhibit a plateau or mesa effect. Propellants exhibiting either a plateau or mesa effect give a constant or decreasing burning rate and as a result a steady thrust. Thus vehicles using such propellants exhibit flatter and more ascertainable trajectories.
Currently, desired plateau burning rates are imparted to propellants by the use of ballistic modifiers which are essentially lead and/or copper salts of aromatic acids.
The present invention consists of a metallic complex formed as the product of the pyrolysis of heavy metal oxides and various selected ureas. The complex formed has been found to be a highly effective ballistic modifier producing plateau ballistic characteristics in a high energy smokeless nitramine plastisol propellant system.
It is an object of this invention to provide an improved ballistic modifier.
A further object of this invention is to provide a novel carbonaceous metal-metaloxide mixture and a process for producing same.
A still further object of this invention is to provide a ballistic modifier which produces plateau burning characteristics in a high impulse nitramine plastisol propellant composition.
Yet another object is to provide a process for producing a ballistic modifier which is inexpensive, less time consuming and does not require strict procedural control.
Other objects and many of the attendant advantages of this invention will be readily appreciated as the same become better understood by reference to the following detailed description.
The present modifier is produced independently of the propellant by mixing a metal oxide and a urea and heating the mixture to an elevated temperature such that a pyrolysis occurs.
A preferred method of effecting the above consists of mixing lead stannate and a urea compound and transferring the mixture to an aluminum container which is covered with aluminum foil. The foil is slit in several places and the container is placed in a muffle furnace at a temperature of between 500.degree. to 550.degree. C. When the initial reaction is completed (smoke no longer evolved), the container is removed and quickly sealed with aluminum foil. The sealed container is again placed in the furnace and after heating at 500.degree. to 550.degree. C for one hour, the container is removed and the contents allowed to cool over night without exposure to air. The pyrolyzed mixture (ballistic modifier) is sieved through a No. 325 mesh screen and is then suitable for incorporation into a plastisol formulation in which plateau burning rates are desired.
The metal compounds which were found to be effective include lead stannate, mixtures of lead oxide (10% to 90%) and stannic oxide (90% to 10%), and mixtures of lead sesquioxide (10% to 30%) and stannic oxide (90% to 70%).
A preferred class of ureas correspond with the general formula ##STR1## wherein X is oxygen or sulphur and R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are the same or different and represent hydrogen, alkyl, cycloalkyl, aryl, alkaryl, arakyl or acyl groups or wherein R.sub.1 and R.sub.4 together represent a divalent organic radical forming a cyclic urea in which the urea moiety is part of the ring.
The above mentioned groups can also have substituents thereon, such as halogen e.g. chlorine, hydroxyl and nitro groups.
Specific examples of ureas which may be used to advantage include urea; alkyl ureas such as methylurea, s-dimethylurea, ethylurea, allylurea, s-diethylurea, n-propylurea, n-s-dipropylurea, tetramethylurea and tetraethylurea; aryl ureas such as phenylurea, s-diphenylurea, tetraphenylurea, and N, N.sup.1 -diethyl-N, N.sup.1 -diphenylurea (ethyl centralite); aralkyl ureas such as benzylurea; acyl ureas such as acetylurea and biuret; hydroxyalkyl ureas such as methylolurea; alkaryl ureas such as p-tolylurea; thiourea; alkyl thioureas such as methylthiourea, ethylthiourea and s-diethylthiourea; aryl thioureas such as phenylthiourea and diphenylthiourea; aralkyl thioureas such as benzylthiourea; and cyclic ureas such as glycolylurea, oxalylurea and malonylurea.
Further, a preferred group of ureas is obtained by reacting selected mono- and polyisocyanates with an excess of water in the presence of a solvent e.g. acetone. Isocyanates which can be used for this purpose include methyl isocyanate, ethyl isocyanate, phenylisocyanate, 2,4 tolyene diisocyanate, hexamethylene diisocyanate, and methylene-di-p-phenylene diisocyanate.
The above mentioned ureas and reaction produced ureas produced suitable results in the pyrolysis reaction of a mixture of selected ureas and heavy metal oxides when 5%-30% by weight, of the urea, was added to lead stannate, chosen mixtures of lead oxide and stannic oxide or chosen mixtures of lead sesquioxide and stannic oxide.
The pyrolysis reaction may be carried out at a variety of temperatures (350.degree.-550.degree. C) with a reaction time between 1-2 hours.
Although it is not intended that the invention be limited thereto, there is set forth herein below for purposes of illustration, examples of how the ballistic modifier may be produced and a propellant composition into which it may be incorporated to provide mesa or plateau burning characteristics.